White blood cells, or leukocytes, make up one of the three types of blood cell. The others are red blood cells and platelets. The leukocyte group contains several different types of cell, each with their own functions and each with a particular appearance under the microscope.
The granular leukocytes are the neutrophils, the eosinophils and the basophils. The agranular cells are the monocytes and the lymphocytes. The basis for the division of the white blood cells into these two groups is the presence of granules inside the granular cells. These granules are observable under a microscope after a sample is treated with a colored stain. A hemotologist or biology student can use the differences in the appearance of the cells and the amount of staining present to identify each cell.
The most common leukocyte is the neutrophil, so called because its small granules do not stain very darkly. It has three to five lobes on its nucleus held together by a substance called chromatin.
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Eosinophils have two lobes on the nucleus and have large granules that stain red/orange. Basophils also have nuclei with two lobes but the large granules stain a dark blue color. Both eosinophils and basophils make up less than a few percent of all white blood cells.
Monocytes have a nucleus shaped like a kidney, and have lots of cytoplasm but no granules visible through a regular microscope, although under an electron scanning microscope, they do contain granules. Lymphocytes have very large, rounded nuclei and little cytoplasm, occasionally with a few granules.
The neutrophil has two different types of granules, which are the lysosomal enzymes and the bacteria-killing enzymes. The cell engulfs foreign invaders and uses these granules to kill them and break them down.
Eosinophils "eat" antigens that antibodies stick to, release a substance toxic to cells and have a regulating effect on the immune system response. Basophils release chemicals such as histamine that cause an inflammatory response.
The purpose of the agranulocyte monocyte is to "eat" up foreign invaders and the remnants of damaged cells. Lymphocytes produce antibodies and can attack invaders.
Under the microscope, when in a fresh blood sample, granular leukocytes have a distinctive method of movement which the agranular leukocytes lack. They extend parts of the cell out in a foot-like manner known as "pseudopodia," so they can move around. They use this ability to move into tissue from the blood to combat infection.